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1
Introduction
1.1. Definition of corrosion Corrosion [1] can be defined as the destruction of metals under the
chemical or electrochemical action of the surrounding environment. All the
corrosion reactions obey the thermodynamic laws. With the exception of noble
metals like gold and platinum all other metals corrode and transform into
substances similar to their mineral ores from which they are extracted. Corrosion
can be simply defined as "the reverse process of extraction of metals".
1.2. Corrosion behavior of zinc in aqueous environment
1.2.1. What is zinc?
1.2.1.1. General properties [2] Zinc has an atomic number of 30 and an atomic weight of 65.38. Its
crystalline structure is hexagonal close-packed which determines physical and
chemical properties. Zinc is an electro-positive element giving up two electrons
for an oxidation state of +2. It is more reactive than brass, steel, nickel or copper
but less so than aluminum or magnesium. The term reactive is relative and
relates more to zinc's position on the electrochemical series at 0.76 volts to the
standard hydrogen electrode.
In a normal atmosphere, zinc forms a basic zinc carbonate film that
greatly retards its corrosion rate, which is similar to the aluminum oxide film
that forms on aluminum which accounts for its low corrosion rate. Zinc when
connected with metals below it in the electrochemical series will sacrificially
protect that metal, which accounts for its wide usage in the galvanized steel
industry.
2
Introduction1.2.1.2. Zinc protects steel in two ways
1. It is a barrier coating, and with its low corrosion rate offers extended life
service.
2. It is sacrificial and protects the steel when exposed to the atmosphere.
The sacrificial situation needs to be considered when coupling zinc to dissimilar
metals.
Zinc is amphoteric and will form positive zinc ions in low pH solutions or
negative zincate ions in high pH conditions and remain relatively insoluble in
the neutral range. The lowest corrosion rates are encountered in the pH range of
6 to 10.5.
1.2.1.3. Zinc corrosion is controlled by four factors
1. Zinc has a high hydrogen overvoltage.
2. Zinc is attacked by acids at rates which decrease as pH increases.
3. Zinc is capable of forming insoluble basic salts when appropriate pH
levels are reached (approximately 8).
4. Zinc is anodic to its metal impurities, to hydrogen and to most surface
contaminants.
The mechanism for zinc corrosion is largely determined by the formation
and stability of the basic carbonate film. Zinc reacts with oxygen to form zinc
oxide (hydroxide) with subsequent reaction with carbon dioxide and water to
form the basic carbonate (ZnO/CO3 Zn (OH) 2). Once this film is formed, the
surface becomes passivated to further attack.
The atmosphere and location in which the material is used as well as
weathering, rates of oxygen diffusion, wet/dry conditions, etc. will all have a
bearing on corrosion rate. The composition of zinc seldom has a significant
3
Introductioneffect on its rate of corrosion in atmospheric exposure. It is probable that all
commercial forms of zinc have within plus or minus 10% of the same corrosion
rate in any given outdoor environment.
1.3. Classification of corrosion
All metal consist of atoms having valency electrons which can be donated
or shared. In a corrosive environment the components of the alloy get ionized
and the movement of the electrons sets up a galvanic or electrochemical cell
which causes oxidation, reduction, dissolution or simple diffusion of the
elements.
The metallurgical approach of corrosion of metals is in terms of the
nature of the alloying characteristics, the phases existing and their inter-
diffusion under different environmental conditions. In fact, the process of
corrosion is a complex phenomenon and it is difficult to predict the exclusive
effect or the individual role involved by any one of the above mentioned
processes.
Based on the above processes, corrosion can be classified in many ways
as low temperature and high temperature corrosion, direct oxidation and
electrochemical corrosion, etc. The preferred classification is:
(i) Dry or chemical corrosion, and.(ii) Wet or electrochemical corrosion.
Dry corrosion occurs in the absence of a liquid phase or above the dew
point of the environment. Vapors and gases are usually coordinates; it is often
associated with high temperature. An example is the attack of steel by furnace
gases .Wet corrosion occurs when a liquid is present in contact with the metal.
This occurs in aqueous solutions or electrolytes. A common example is
corrosion of steel by water.
4
Introduction
The nature and extent of corrosion depend on the metal and the environment.
The important factors which may influence the corrosion process are:
(i) Nature of the metal, (ii) nature of the environment,
(iii) Electrode potential, (iv) temperature,
(v) solution concentration, (vi) aeration,
(vii) agitation, (viii) pH of the solution and
(ix) nature of the corrosion products.
1.3.1. Various forms of corrosion [3]
Corrosion can manifest itself in the following main forms:
1.3.1.1. General corrosion or Uniform attack
This is the most common type in which the corrosion is uniform over the
entire exposed surface. an example of this is water tank exposed to the
atmosphere.
1.3.1.2. Pitting or Localized attack
It is one of the most destructive and insidious forms of corrosion. It is a
highly localized corrosion; the attack is being limited to extremely small areas.
An example is the corrosion of stainless steels in chloride solutions.
1.3.1.3. Galvanic corrosion
It is an accelerated electrochemical action due to two different metals
being in electrical contact and exposed to an electrolyte. Heat exchanger failure
in which aluminum tubes are supported by a perforated steel sheet is an example
of this type of corrosion.
5
Introduction 1.3.1.4. Crevice corrosion
This type of corrosion takes place when only one metal is in contact with
different concentrations of the environment. Rectangular metal containers and
reverted lap joints offer the possibility for this type of corrosion.
1.3.1.5. Stress corrosion
It is the spontaneous cracking resulting from the combined effect of
prolonged stress and corrosive attack. Caustic embrittlement of boilers provides
an example for this type of corrosion.
1.3.1.6. Erosion – corrosion
It is the acceleration in the rate of attack of a metal because of the
relative movement between a corrosive fluid and the metal surface. Heat
exchanger tubes with water movement undergo this type of corrosion.
1.3.1.7. Fretting corrosion
It is a case of deterioration resulting from repetitive rumbling at the
interface between two surfaces in a corrosive environment. It is found in aircraft
engine parts.
1.3.1.8. Filiform corrosion
It is a special type of rusting which occurs on certain metals under protective
films like paints and is characterized by a thread like growth. Filiform corrosion
may be found on tools coated with oil films, refrigerator doors etc.
6
Introduction1.4. Corrosion inhibitors
1.4.1. Definition
An inhibitor is "A substance which retards corrosion when added to an
environment in small concentrations"[4]. Inhibitor may also defined on
electrochemical basis as substances that reduce the rates of either or both of
partial anodic oxidation/ or cathodic reduction reaction. Also corrosion
inhibitors are chemical substances which added in small amounts to the
environment will reduce the corrosion rate of the metals and their alloys [5-7].
1.4.2. Classification of corrosion inhibitors
The corrosion inhibitors are classified according to the environment,
mode of action and composition of the inhibitors. The inhibitors are classified as
acidic inhibitors, neutral inhibitors, alkaline inhibitors, and vapor phase
inhibitors according to the environments in which they find applications
Inhibitors can also be classified as anodic [8, 9], cathodic [10], and
mixed-type [11] inhibitors depending the mechanism of their inhibiting action
on the electrochemical corrosion. Other classifications like passivators,
precipitate inhibitors and adsorbents are also in use. Depending on the nature of
the inhibitors they can also classified as organic or inorganic inhibitors [12-14].
The different modes of inhibiting electrode reactions had been analyzed by
Fischer [15] and distinguished among various mechanisms of action such as
(i)interface inhibition, (ii) electrolyte layer inhibition, (iii) membrane inhibition,
and (iv) passivation .Subsequently, Lorenz and Mansfield proposed a clear
distinction between interface and interphase inhibition. They proposed two
different types of retardation mechanisms of electrode depending on types of
reaction including corrosion. Interface inhibition presumes a strong interaction
between the inhibitors and the surface of the metal. In this case the inhibitor
adsorbs as a potential dependent two-dimensional layer. Interphase inhibition
7
Introductionpresumes a three dimensional layer between the corroding substrate and the
electrolyte [16-20]. Such 3-dimensional layer generally consists of weakly
soluble compounds such as oxidized corrosion products or inhibitor forming
porous layers .The inhibition efficiency depends strongly on the three
dimensional layers especially its porosity and solubility.
1.4.3. Corrosion rate measurements
Recently the measurement of corrosion rates in the presence of
corrosion inhibitors by weight-loss and electrochemical methods have been
reviewed by Mercer [21].
Chemical method1.4.3.1.Weight-loss measurements)i
The efficiency of inhibition was calculated from the weight loss values
using the following equation:
% IE = 100xW
WW
free
.addfree
(1.1)
where, Wfree = weight loss in the corrosive medium in mg cm-2.
Wadd. = weight loss in the inhibited solutions in mg cm-2.
ii) Hydrogen evolution method
The efficiency of a given inhibitor can be evaluated as the percentage
reduction in reaction rate (K), so the percentage inhibition efficiency (% IE) can
be calculated, as follows:
100xK
KKIE%free
addfree
(1.2)
where, Kfree = specific reaction rate in the corrosive medium. Kadd. = specific reaction rate in the inhibited solutions.
Specific reaction rate constants are calculated from the relation:V = Kt (1.3)
8
Introductionwhere, V = volume of hydrogen evolved in cm3.
t = time in min.iii) Thermometric method
A simple rapid and limited method for comparing the inhibition
efficiency of different additional agents [22]. A reaction number, R.N., is
defined by Mylius as:
min /Ct
T-T.N.R im
(1.4)
where, Tm = maximum temperature in K.,
Ti = initial temperature in K.
t = time in min. from the start of the experiment to attained Tm.
The reaction number is proportional to the rate of the corrosion of the
metal. The extent of corrosion inhibition by a certain concentration of a
particular additive is evaluated from the percentage reduction in the reaction
number
% Reduction in R.N. = 100.).(
.).((R.N.) .free xNR
NR
free
add (1.5)
where, (R.N.)free = reduction in R.N. in the corrosive medium.
(R.N.)add. = reduction in R.N. in presence of additive.
1.4.3.2. Electrochemical methods
i) Open-circuit potential method
This method is used to measure the steady state potential, (EOCP), of the
metal or alloy in the absence and presence of additives [23]. In this method, the
potential of the corroding material is measured against reference electrode at
different time intervals until a steady state is reached.
9
Introductionii) Potentiodynamic polarization method
a) Tafel plots
In this method corrosion current can be determined from polarization
curves by intercept method based on anodic and/or cathodic Tafel curves.
The intercept method [24, 25] used in the present work for the
determination of corrosion current is performed by extrapolating the Tafel lines
to the experimentally was measured free corrosion potential. The corresponding
log icorr. value was obtained by interception and icorr. then evaluated.
b) Linear polarization method
This technique is an accepted method of monitoring corrosion rates
[26-29]. For a corroding electrode, the polarization resistance Rp, at small
applied range of potential or current, is related to the corrosion current density,
and icorr. calculated by the following equation
icorr
=ppca
ca
RB
R1.
)b(b2.3bb
(1.6)
where, ba = anodic Tafel slope.
bc = cathodic Tafel slope.
Rp = is the polarization resistance.
A controlled potential scan is applied over a small range, typically
25 mV with respect to Ecorr. at a scan rate of 0.1 mV/sec. The resulting current is
plotted against the potential. The slope of this linear potential – current plot at
Ecorr. is identical with the polarization resistance which used together with the
measured Tafel constants to determine the corrosion rate.
The percentage inhibition efficiency (%IE) can expressed as:
10
Introduction100x
iiiIE%
free
.addfree
(1.7)
where, ifree = corrosion current in the corrosive medium. iadd. = corrosion current in presence of additive.iii) Impedance method
In recent years, the impedance technique has been widely used for
measuring the corrosion rates of metals. The main advantage of this method is
the complete elimination of the solution resistance. The equivalent circuit of a
corroding metal which has both anodic and cathodic reactions under activation
control may be represented as a parallel combination of charge transfer
resistance (Rt) and double layer capacitance (Cdl) and the solution resistance (Rs)
is in series.
From the Rt value, the icorr is obtained from the relationship:
tca
ca.corr Rx)b2.303(b
b xbi (1.8)
The impedance of the above circuit for the given ( = 2 f) is:
tdl
s
R1Cj
1RZ
(1.9)
2t
2dl
2
2tdl
2t
2dl
2t
RC1RCj
RC1RZ
(1.10)
Z = Z' – jz" (1.11)
This method gives the instantaneous corrosion rate. This technique canbe used in low and high resistive media.
Two main plots formats are frequently used in impedance due topresentation, the Nequist and Bode plots. There are also some other plots whichenable interpretation of diffusion phenomena or specific electrochemical process[30].
11
Introductiona) The Nyquist plot
In this plot the imaginary component, Z" is plotted against the real
component, Z' at each excitation frequency. But the frequency is not presented
on the plot. In the ideal case the plot is in the form of semicircle as shown in Fig.
(1.1), from this figure, at high frequencies, the impedance of the Randles cell
was almost entirely created by the ohmic resistance Rs or R , the frequency
reaches its high limit at the left most of the semicircle, where the semicircle
touches the x axis (real component). At the low frequency limit, the Randles cell
also approximates a pure resistance, but the value is (Rs + Rp). The frequency
reaches its low limit at the right most end of the semicircle. Rs can be
determined from the frequency independent limit of Z at high frequencies and
(Rs+Rp) from the corresponding limit at low frequencies.
The double layer capacitance, Cdl can be determined from the value of
the angular frequency at maximum Z" ( max) and the polarization resistance
according to:
pmaxpmaxdl Rf2
1R
1C (1.12)
Fig. (1.1): Nyquist plot with impedance vector.
12
Introduction The figure shows that low frequency data are on the right side of the
plot and higher frequencies are on the left. This is true for EIS data where
impedance usually falls as frequency rises (this is not true of all circuits).
b) Bode plot
The impedance is plotted with log frequency on the x-axis and both the
absolute value of the impedance (|Z| =Z0 ) and phase-shift on the y-axis. Unlike
the Nyquist plot, the Bode plot explicitly shows frequency information as
shown in Fig.(1.2).
Fig.(1.2): Bode Plot with One Time Constant
In this plot, the absolute impedance Z , as calculated by the equation:
Z = [(Z')2 + (Z")2]0.5 (1.13)
And the phase shift, o, are plotted as a function of the frequency, f. the log Z
vs. log f presentation had some distinct advantage over the Nyquist plot. It
enables a wide range of measurements over orders of frequency magnitude
(from 10-2 to 105 Hz) and shows the dependence of the electrode impedance on
the frequency on a direct look. The log Z vs. log f curve can yield values of
Rp and Rs. At the highest frequencies shown in Fig.(1.2) the ohmic resistance
13
Introductiondominates the impedance and log Rs can be read from the higher frequency
horizontal plateau at the lowest frequencies, polarization resistance also
contributes and log (Rp + Rs) can be read at the low frequency horizontal
plateau. At intermediate frequencies, this curve is a straight line with a slope of
(-1). Extrapolating this line to log Z axis at = 1 (log = o at f = 0.16 Hz)
yields the value of Cdl from the equation:
Z 0.16HzdlC
1 (1.14)
The Bode plot also shows the phase angle, . At the high and low
frequency limits, where the behavior of Randles cell is resistor-like, the phase
angle is nearly zero. At intermediate frequencies, , increases as the imaginary
component of the impedance increase. The vs. log f plot yields a peak at
which the phase shift of the response is maximum.
To calculate the corrosion rate, one must determine the corrosion
current (icorr), and to determine icorr. From polarization resistance, the Tafel
constants are needed. Equation (1.8) shows the relationship between the Rt
value, the Tafel constants and the corrosion current.
vi) Electrochemical frequency modulation method
Electrochemical frequency modulation (EFM) is a new technique
provides a new tool for electrochemical corrosion monitoring. In this
technique, a potential perturbation by two sine waves of different frequencies
is applied to a corroding system. As corrosion process is non- linear in nature,
responses are generated at more frequencies than the frequencies of the applied
signal. The current responses can be measured at zero, harmonic, and inter
modulation frequencies.
14
Introduction Analysis of these current responses can result in the corrosion current
density and Tafel parameters. Electrochemical frequency modulation is
nondestructive technique as electrochemical impedance (EIS) that can directly
and rapidly give values of the corrosion current without a prior knowledge of
Tafel constants. The great strength of the EFM is the causality factor, which
serves as an internal check on the validity of the EFM measurement. With the
causality factors the experimental EFM data can be verified [31,32]. Identical
cell assembly as used in both polarization and impedance studies was used for
Electrochemical frequency modulation (EFM) measurement. Experiments were
carried out using potential amplitude 10mV for both frequencies 2 and 5 Hz. All
electrochemical experiments were carried out using Gamry PCI300/4
Potentiostat / Galvanostat / Zra analyzer, DC105 Corrosion software, EIS300
Electrochemical impedance spectroscopy software, EFM140 electro chemical
frequency modulation software and echem analyst 5.21 for results plotting,
graphing, data fitting & calculating.
1.5. Literature Survey on Corrosion Inhibition of Zinc in Aqueous
Solutions
The inhibiting action of 2-mercaptobenzimidazole on the corrosion of
zinc in phosphoric acid (H3PO4) solution was investigated by weight loss and
polarization [33]. The result reveals that the inhibitor is effective for the
inhibition of zinc in H3PO4 solution and retards the anodic and cathodic
corrosion reactions with emphasis on the former ( mixed inhibitors).
Ethoxylated fatty alcohols with different numbers of ethylene oxide
units were tested as corrosion inhibitors for dissolution of zinc in hydrochloric
acid using weight loss and galvanostatic polarization measurements [34] .The
inhibition efficiency was found to increase with increasing concentration,
15
Introductionnumber of ethylene oxide units per molecule and with decreasing temperature.
Inhibition was explained on the basis of adsorption of ethoxylated fatty
alcohols molecules on the metal surface through their ethoxy group. The
degree of surface coverage varied linearly wilh logarithm of inhibitor
concentration Fitting Temkin isotherm.
The action of some organic compounds from the group of surfactants
and polyethylene glycols (PEGs) on the corrosion of zinc in alkaline media was
studied [35]. The investigations were carried out by use of electrochemical and
nonelectrochemical methods. The effectiveness of the inhibitors was compared
and it was found that the PEG of average molecular weight 400 (PEG400) was
especially effective. The assumption has been advanced that zinc corrodes
electrochemically in the first stage of exposition, but the chemical corrosion
prevails after a longer time.
The inhibition of zinc corrosion in an aerated 0.5 M NaCl solution by
environmentally acceptable cation inhibitors, Al3, La3, Ce3+and Ce4+ was studied
[36]. The inhibition efficiencies of these cations were determined by
polarization measurements of a zinc electrode in the solution. The efficiencies of
Ce3+ and La3+ were high, more than 90% but those of other cations were
negative, indicating stimulation of zinc corrosion. X-ray photoelectron spectra
for the zinc surface treated in the solution containing Ce3+ revealed that a thick
protective layer composed of Ce(OH)3, Ce2O3 and small amounts of Zn(OH)2
and ZnO formed on the surface and there was no chloride ion within the layer.
Processes of the layer formation on the surface were discussed based on these
results.
Cerium(III) chloride ( CeCl3) is an effective inhibitor for corrosion of
zinc in an aerated 0.5 M NaCl solution by the formation of a thick film
composed of cerium-rich oxide and hydroxide [37]. In this study, a protective
16
Introductionfilm was prepared by treatment of a zinc electrode in an aqueous solution of
1×10-3 M Ce (NO3)3 at 30°C for 30 min and examined in an aerated 0.5 M NaCl
solution at 30°C by polarization measurements after immersion of the electrode
in the NaCl solution for 4–240 h. The film comprised oxides and hydroxides of
Ce (III) and Ce (IV) ions mostly. The protective efficiency of the film against
zinc corrosion was maintained remarkably high, more than 91% during
immersion in 0.5 M NaCl for 240 h. Little pitting corrosion was observed on the
electrode surface. However, this film could not heal the scratched surface of zinc
electrode in the NaCl solution.
Effects of some organic inhibitors e.g. [sodium benzoate (NaBz),
sodium N-dodecanoylsarcosinate (NaDS), sodium S-octyl-3-thiopropionate
(NaOTP), 8-quinolinol (8-QOH) and 1,2,3-benzotriazole (BTAH)] on corrosion
of zinc in an aerated 0.5 M NaCl solution were investigated by polarization
measurements [38]. The inhibition was explained in terms of formation of
precipitate films of Zn(II) salts or complexes on the zinc surface together with
zinc hydroxide and oxide to prevent corrosion in the solution. High inhibition
efficiencies of NaOTP were acquired at concentrations C =1×10-6–3×10-5 M,
BTAH at C =1×10-4–1×10-3 M, 8-QOH at C =1×10-3–3×10-3 M and NaBz at C
=3×10-3–1×10-2 M. The film formed on the zinc surface was analyzed by X-ray
photoelectron and Fourier transforms infrared reflection spectroscopies.
New surface treatment of zinc, called "carboxylting", which could be
an environmentally friendly alternative to the usual conversion treatments [39].
Carboxyl ting requires use of both n-alkyl carboxylic acid and peroxoborate as
oxidizing agent, in a water–ethanol mixture. As in the phosphating process, the
carboxylating one is carried out in four steps: the activation step, oxidation of
the zinc substrate at an acidic pH, germination and growth of zinc carboxylate
crystals. Influence of the activator content, treatment time, oxidizing agent
17
Introductionconcentration, and chain length of the carboxylic acid was successively
examined.C arboxylates covered zinc samples were tested in corrosion
conditions. The longer the carbon chain length, the more corrosion resistant the
coating; its layered structure easily allows exchanging anions and a
hydroxocarboxylate of zinc is stabilized during the corrosion test then delays
the white rust formation.
The synergistic inhibition effects of cerium(III) chloride, CeCl3 and
sodium silicate, Na2Si2O5 (water glass) on corrosion of zinc in an aerated 0.5M
NaCl solution at 30ºC were examined by polarization measurements [40].
Equimolar mixtures of these inhibitors were markedly effective, indicating that
the inhibition efficiencies of the mixture at 1x10-4 of each inhibitor were 95.9 %
and 93.6 % after immersion of a zinc electrode in the solution for 3 and
120h.,respectively. X-ray photoelectron spectra revealed that a protective layer
composed of hydroxylated or hydrated cerium-rich oxide and small amounts of
zinc hydroxide and silicate formed on the zinc surface. A high inhibition
efficiency of 1x10-3 M Na2Si2O5, 97.7 % was obtained for corrosion of a zinc
electrode which was previously treated in the NaCl solution of 1x10-3 M CeCl3
at 30ºC for 30 min.
Cerium(III) chloride,CeCl3 and sodium octythiopropionate
C8H17S(CH2)2COONa (NaOTP) are effective inhibitors for zinc corrosion in
0.5M NaCl. Synergistic inhibition of zinc corrosion in an aerated 0.5M NaCl
solution by a mixture of these inhibitors was investigated by polarization
measurements after immersion of a zinc electrode in the solution for many hours
[41]. The inhibition efficiency of 1x10-4M CeCl3 plus 1x10-5M NaOTP mixture
was high, 95.1% after both 3h. and 120 h. X-ray photoelectron spectroscopy and
electron –probe microanalysis for the inhibited electrode revealed that the zinc
surface was covered with a protective film composed of a hydrated or
18
Introductionhydroxylated Ce-rich oxide, a small amount of Zn (OH) 2 and a trace of Zn
(OTP) 2 chelate. The inhibition effect of 1x10-5M NaOTP in the NaCl solution
for the zinc electrode previously treated in 1x10-3 M CeCl3 for 30 min was also
examined, indicating higher inhibition efficiency, 96.3% after immersion of the
electrode in the solution for 120h.
A self-assembled monolayer (SAM) of hexadecanoate ion C15H31CO2-
(C16A-) was prepared on a zinc electrode covered with a layer of hydrated
cerium(III) oxide Ce2O3.The protection of zinc against corrosion was examined
for the electrode coated with the Ce2O3 layer and the C16A- SAM in an
oxygenated 0.5M NaCl solution [42]. A more positive open-circuit potential of
the coated electrode was maintained during immersion in the solution for 4h.
than that of the uncoated one and polarization curves showed marked
suppression of the anodic process, implying that the layer was extremely high,
more than 99%. The zinc surface coated with the Ce2O3 layer and the C16A-
SAM was characterized by X-ray photoelectron and FTIR reflection
spectroscopies and contact angle measurement with a drop of water.
The influence of oxide surface charge on the corrosion performance of
zinc metals was investigated [43]. Oxidized zinc species ( zinc oxide, zinc
hydroxyl chloride, zinc hydroxyl sulfate and zinc hydroxyl carbonate) with
chemical compositions similar to those produced on zinc during atmospheric
corrosion are formed as particles from aqueous solution, and as passive films
deposited onto zinc powder, and rolled zinc surfaces. Synthesized oxides are
characterized by X-ray diffraction, Fourier transform infrared spectroscopy,
scanning electron microscopy and electron probe X-ray micro analysis. The zeta
potentials of various oxide particles, are determined by micro electrophoresis,
are reported as a function of pH. Particulates containing a majority of zinc
hydroxyl carbonate and zinc hydroxyl sulfate crystallites are found to possess a
19
Introductionnegative surface charge below pH 6, whilst zinc oxide- hydroxide and zinc
hydroxyl chloride crystallites possess isoelectric points (IEP's) higher than pH
8.The ability of chloride species to pass through a bed of 3 ml diameter zinc
powder is found to increase for surface possessing carboxyl and sulfate surface
species, suggesting that negatively charged surface can aid in the repulsion of
chloride ions. Electrochemical analysis of the open-circuit potential as a
function of time at a fixed pH 6.5 showed that the chemical composition of
passive films on zinc plates influences the ability of chloride ions to access
anodic sites for periods of approximately1h.
The influence of alloying elements on the corrosion behavior of rolled
zinc sheet in aqueous media has been investigated by means of electrochemical
techniques [44]. All the change in corrosion behavior seen in this study could be
attributed to modification of the formation or stability of the passivating oxide
film on the zinc surface. A low concentration of copper (0.6wt.%) inhibits the
formation of the passivating film and reduces the stability of the film.
Conversely, a low concentration of chromium (0.5wt.%) accelerates the
passivation process and raises the stability of the film. The passivation and
corrosion behavior shown by a commercially produced ternary alloy containing
copper and titanium additions is almost the same as the behavior shown by
model binary alloy containing only copper. The results obtained in this study are
consistent with the hypothesis that alloying elements alter the electron-
conducting and /or ion –conducting properties of the passivating oxide film.
The corrosion rate of zinc metal in aqueous solutions of potassium
hydroxide was followed in time by measuring the amount of evolved hydrogen
[45]. The zinc samples were pretreated with either mono benzyl amine, dibenzyl
amine, tri benzyl amine or sulphonated benzyl amine.The hydroxide
concentration was varied from 1 to 10 N. It was shown that the amines act either
20
Introductionas corrosion inhibitors or activators, depending on the KOH concentration and
the duration of exposure which lasted up to 150 hrs.
Effects of NaCl and NH4Cl on the initial atmospheric corrosion of
zinc are investigated via quartz crystal microbalance ( QCM ) in laboratory at
80% RH { Relative Humidity} and 25°C [46]. The results showed that both
NaCl and NH4Cl can accelerate the initial atmospheric corrosion of zinc. The
combined effect NaCl and NH4Cl on the corrosion of zinc is greater than that
caused by NH4Cl and less than that caused by NaCl. Fourier transform infrared
spectroscopy (FTIR), X-ray diffraction( XRD), Scanning electron microscopy
(SEM) and Electron dispersion X-ray analysis (EDAX) are used to characterize
the corrosion products of zinc. (NH4)2ZnCl4, Zn5(OH)8Cl2.H2O and ZnO present
on zinc surface in the presence of NH4Cl while Zn5(OH)8Cl2.H2O and ZnO are
the dominant corrosion products on NaCl treated zinc surface.
A comparison between the potentiodynamic behavior of the stationary
and the rotating Zn disc electrodes in naturally aerated and de- aerated 0.1 M
KClO4 solution is performed [47]. The voltammograms of the stationary
electrode in both solutions exhibited one anodic peak and two cathodic peaks.
The anodic peak is replaced by two mass transport controlled O2 reduction
cathodic current plateaus in the forward scan by rotating the electrode in
naturally aerated solutions. However, the reverse scan is characterized by only
one cathodic peak observed at a potential depends on the experimental
conditions. The more cathodic reduction peak referred to the reduction of the
passive layer and split into two peaks at low scan rates. Interpretation of
these data is made adopting a multi-path mechanism and a two layer
passive film model. A correlation between the ClO4- and dissolved O2
reduction and the thickness of the two passive layers is performed. The
protective nature of the passive layers forms in different experimental conditions
21
Introductionis found to decrease with rotating the electrode and de-aerating the solution.
Chronopotentiometry and electrochemical impedance measurements are also
used in this study .Impedance technique shows a change in the ZnO thickness
with the experimental conditions as a result of changing the reactions occurring
in the electrode vicinity.
The influence of some hydrazide derivatives as corrosion inhibitors for
zinc in 2M sodium hydroxide solution has been studied using weight loss and
galvanostatic polarization technique [48].In general, at constant acid
concentration, the inhibitor efficiency increases with increase of concentration
of inhibitor and decrease with rise in temperature. Polarization studied reveals
that these compounds behave as mixed inhibitors. The effect of temperature was
studied and activation energies were calculated and some thermodynamic
activation parameters are calculated and discussed. The inhibition process can
be explained in view of adsorption on the zinc surface. The adsorption of the
inhibitors on zinc surface is found to obey Temkin adsorption isotherm.
Addition of Ca2+, Sr2+, Ba2+ and Mg2+ ions to the alkaline medium containing the
hydrazide derivatives increases the inhibition efficiency of the system.
The corrosion inhibition of zinc in hydrochloric acid by extract of
Nypa Fruticans Wurmb was studied using weight loss techniques[49].
Maximum inhibition efficiency (and surface coverage) is obtained at an
optimum concentration. However increase in temperature decreases the
inhibition efficiency. The inhibition action of Nypa Fruticans Wurmb extract
compared closely to that of 1,5-Diphenyl Carbazone (DPC).Optimum inhibition
efficiency for zinc in the presence of Nypa Fruticans Wurmb extract is 36.43%
and 40.70% with DPC. The phenomenon of physical adsorption has been
proposed from the activation energy values (19.33 kJ.mol-1 and 21.11 kJ.mol-1)
22
Introductionwith Nypa Fruticans Wurmb extract and DPC respectively. A first order kinetics
has been deduced from the kinetic treatment of the results.
The zinc metal surface is chemically modified by newly synthesized
Schiff’s bases and its corrosion protection was investigated [50]. The influence
of increasing concentration of Schiff’s bases on modification of zinc surface
and immersion time in treatment bath are investigated and optimized for
maximum corrosion protection efficiency. The electrochemical studies of treated
zinc specimens are performed in aqueous acid solution using galvanostatic
polarization technique. The treated zinc samples show good corrosion
resistance. The recorded electrochemical data of chemically treated samples
indicate a basic modification of the zinc surface. The protection efficiency of
organic layer formed on zinc surface is tested by varying the acid concentration
and temperature of the corrosive medium. The corrosion protection efficiency
increases with the concentration of Schiff’s bases and immersion time. This is
due to a strong interaction between zinc and the organic molecules, which
results in the formation of a protective layer? This layer prevents the contact of
aggressive medium with the zinc surface. The surface modification is confirmed
by the scanning electron microscopy images. The interaction between metal
atoms and Schiff’s bases is also established by IR studies.
The surface treatment of zinc and its corrosion inhibition was studied
[51] using a product formed in the reaction between benzaldehyde and thiosemi-
carbozide (BTSC). The corrosion behavior of chemically treated zinc surface
was investigated in aqueous chloride-sulphate medium using galvanostatic
polarization technique. Zinc samples treated in BTSC solution exhibited good
corrosion resistance.The measured electrochemical data indicated a basic
modification of the cathode reaction during corrosion of treated zinc.The
corrosion protection may be explained on the basis of adsorption and formation
23
Introductionof BTSC film on zinc surface. The film was binding strongly to the metal
surface through nitrogen and sulphur atoms of the product. The formation of
film on the zinc surface was established by surface analysis techniques such
as scanning electron microscopy (SEM-EDS) and Fourier transform infrared
spectroscopy (FTIR).
The electrochemical behavior of pure Zn and galvanized steel in
solutions simulating the pore solution of carbonated concrete was studied by
means of potentiodynamic polarization tests and polarization resistance
measurements [52]. Pure Zn was chosen because it simulates well the behavior
of galvanized steel, yielding more reproducible results. The effect of different
degrees of carbonation and the presence of different chloride contents in the
simulated pore solutions was investigated. Results showed that at a given pH
(about 9.5) the corrosion susceptibility of Zn depends on anions concentration
(carbonate and bicarbonate). Results obtained in simulated carbonated concrete
Pore solutions showed that with low anion concentration Zn did not passivate
while in presence of high levels of carbonate and bicarbonate the corrosion
resistance is improved. Besides, the presence of chloride increased the corrosion
susceptibility.
The corrosion inhibition behavior of Neem (Azadirachta indica) nature
leaves extract as a green inhibitor of zinc corrosion in 2.0N hydrochloric acid
( HCl ) solutions has been studied using gravimetric and thermometric
techniques for experiments conducted at 30°C and 60°C [53]. The results
revealed that different concentrations of the Azadirachta indica (AZI) extract
inhibit zinc corrosion and that inhibition efficiency of the extract varies with
concentration and temperature. For extract concentrations studied and ranging
from 9.09–23.08 mg / L, the maximum inhibition efficiency was 78.33% and
68.95% both at 23.08 mg / L AZI at 30°C and 60°C, respectively.
24
Introduction The corrosion inhibition properties of Ocimum tenuiflorum (Tulsi)
leaves extract as a potential green inhibitor of zinc corrosion in H2SO4 have been
investigated using gravimetric and thermometric techniques [54]. The results
show that different concentrations of the Tulsi extract have inhibited zinc
corrosion and that the inhibition efficiency had varied with the concentration of
extract and temperature of experimental corrosion half-cell.
The effect of sodium eperuate prepared from Wallaba ( Eperua
falcata Aubl) extract on zinc corrosion was investigated in alkaline
solutions with chloride ions ( i.e., simulated concrete pore solutions ) by
using electrochemical techniques [55]. Sodium eperuate inhibits the corrosion
of zinc in 0.1 M NaCl solutions with pH 9.6. As its concentration increases to 1
g/L, the inhibition efficiency reaches approximately 92 %. In alkaline solutions
with pH 12.6, sodium eperuate has no adverse effect on passivity of zinc, and
retards the chloride attack. These suggest that sodium eperuate is an effective
inhibitor for the protection of zinc in alkaline environments.
The aqueous extract of the leaves of henna ( lawsonia ) is tested as
corrosion inhibitor of C-steel, nickel and zinc in acidic , neutral and alkaline
solutions, using the polarization technique [56]. It was found that the extract
acts as a good corrosion inhibitor for the three tested electrodes in all
tested media. The inhibition efficiency increases as the added concentration of
extract is increased. The degree of inhibition depends on the nature of
metal and the type of the medium. For C-steel and nickel, the inhibition
efficiency increases in the order: alkaline < neutral < acid, while in the case of
zinc it increases in the order: acid < alkaline < neutral. The extract acts as a
mixed inhibitor. The inhibitive action of the extract is discussed in view of
adsorption of lawsonia molecules on the metal surface. It was found that
this adsorption follows Langmuir adsorption isotherm in all tested systems.
25
IntroductionThe formation of complex between metal cations and lawsone is also
proposed as additional inhibition mechanism of C-steel and nickel
corrosion.
The inhibition of the corrosion of zinc by acetone extract of red
onion skin in hydrochloric acid solutions has been studied using weight loss
method [57]. The results of the study reveal that different concentrations of
the extract inhibit zinc corrosion. Inhibition efficiency of the extract is found
to vary with concentration and temperature. The active component in red
onion skin is quercetin. Acetone extract of red onion skin could serve as
an effective and non-toxic inhibitor of the corrosion of zinc in hydrochloric
acid solution.
The inhibitive effect of fenugreek (Trigonella foenum graecum) seeds
extract on the corrosion of zinc in aqueous solution of 0.5 M sulphuric
acid were investigated at 30,35,40, and 45°C by potentiodynamic polarization
and electrochemical impedance spectroscopy (EIS) techniques [58].
Potentiodynamic polarization curves indicated that the fenugreek seeds
extract behaves as an anodic type inhibitor. EIS measurements showed that
the dissolution process occurs under activation control .Inhibition was
found to increase with increasing concentration of the fenugreek seeds
extract but decreased with the increase of the temperature. The associated
activation parameters were determined. Results showed that the fenugreek
seeds extract could be used as an effective inhibitor for the corrosion of
zinc in sulphuric acid media at higher temperature .
The effect of the extract of Aloe vera leaves on the corrosion of
zinc in 2 M HCl solution was studied using weight loss technique [59]. A
vera extract inhibited the corrosion of zinc in 2 M HCl solution and the
26
Introductioninhibition efficiency increased with increasing concentration of the extract but
decreased with increasing temperature. The adsorption of the inhibitor
molecules on zinc surface was in accordance with Langmuir adsorption
isotherm. A first-order kinetics relationship with respect to zinc was obtained
with and without the extract from the kinetics treatment of the data.
The efficiency of some substituted N-arylpyrroles as zinc corrosion
inhibitors in hydrochloric acid was examined by electrochemical (d.c. and a.c.)
and gravimetric methods [60]. The influence of the structure and composition of
a molecule on the inhibition characteristics was observed by investigation of the
action of the functional group located on the pyrrole ring (-CHO) and at the
ortho position of the benzene ring (-H, -Cl, -CH3). The results have shown that
all the organic compounds investigated possess good inhibiting properties. In
contrast to most commercial acid corrosion inhibitors, which are highly toxic
and very hazardous products, substituted N-arylpyrroles are nontoxic
compounds with good environmental characteristics.
The effect of some amidopolyethylamine, with different numbers of
ethylamine units on the corrosion of zinc electrode in ZnCl2, NH4Cl and (ZnCl2
+ NH4Cl) electrolytes had been studied using galvanostatic polarization
measurements [61]. The inhibition efficiency was found to increase with
increasing concentration, number of ethylamine units per molecule and with
decreasing the temperature. Inhibition was explained on the basis of adsorption
of amidopolyethylamine molecules on the zinc electrode surface through their
ethylamine groups. The inhibitors are adsorbed on the zinc electrode surface
according to Langmuir adsorption isotherm. Some thermodynamic parameters
are calculated and explained for the tested systems from the obtained data at
different temperatures.
27
Introduction The corrosion behavior of zinc electrode in (HNO3 + HCl) binary acid
mixture containing ethylamines was investigated [62]. In binary acid mixture,
the corrosion rate increased with concentration of mixture acid and with the
temperature. At constant mixture acid concentration, the inhibition efficiencies
of ethylamines increase with the inhibitor concentration. Similarly, at constant
inhibitor concentration, the inhibition efficiency increases with the increase in
concentration of mixture acid. At IE % inhibitor concentration in (0.01 N HNO3
+ 0.01 N HCl) acid mixture at 301 K for 24 hr immersion period, the inhibition
efficiency decreases in the order: ethylamine (98%) > diethylamide (95%) >
triethylamine (91%). As temperature increases, the value of free energy G*
increased, while percentage of inhibition decreases. The mode of inhibitor action
appeared to be chemisorption since the plot of log ( /1- ) versus log C gave a
straight line which suggests that the inhibitors cover both the anodic and
cathodic regions through general adsorption following the Langmuir isotherm.
Anodic and cathodic galvanostatic polarization curves showed little anodic but
significant cathodic polarization.
The evaluation of Schiff bases derived from o-, m- and p-amino
phenols and salicylaldehyde as corrosion inhibitors of zinc in sulfuric acid and
to study their inhibitor mechanism [63]. The effect of various parameters on the
behavior of these inhibitors has been studied using the weight loss and
polarization measurements. In general, the ortho isomer was highly effective as
a corrosion inhibitor because it forms a chelate with a six-membered ring and
moreover the ortho isomer possessed pronounced electromeric effect. These
inhibitors obey the Langmuir adsorption isotherm. The almost constant
performance with temperature in the case of ortho and para isomers in 0.5M
sulfuric acid suggested strong adsorption bonds. The thermodynamic parameters
suggested that this strong interaction of the inhibitor molecules with the metal
surface resulted in spontaneous adsorption. It may be concluded that a good
28
Introductioninhibitor is characterized by a relatively greater decrease in free energy of
adsorption, lower entropy of adsorption and higher heat of adsorption.
Polarization data indicated that all these isomers were predominantly cathodic
inhibitors. The conjoint effect of external cathodic current and these inhibitors
was either synergistic or additive.
The anodic behavior of Zn electrode in 1 × 10 2 M Na2B4O7 solutions
in the absence and presence of various concentrations of Na2SO4, Na2S2O3 or
Na2S as aggressive agent was studied by galvanostatic polarization technique
[64]. In the absence of sulphur-containing anions in solution, the polarization
curves are characterized by one distinct arrest corresponding to Zn(OH)2 and/or
ZnO, after which the potential increased linearly with time due to the formation
of barrier oxide film before reaching the oxygen evolution reaction. The
duration time of the arrest decreased with increasing current density while the
rate of oxide film formation increases. On the other hand, the duration time of
the arrest increases with the number of anodic cyclization while the rate of oxide
film formation decreased. Additions of low concentration of the aggressive
anions have no effect on the passive film formed on the metal surface. The
potential starts to oscillate within the oxygen evolution region with increases in
the concentration of these aggressive anions. Further increases in the
concentration of these aggressive anions are associated with impaired Zn
passivity that might indicate pitting attack. The aggressiveness of the sulphur
species decreases in the order: . The effect of raising pH of
the solution on the anodic behavior of Zn electrode in the presence of
anions was also investigated. It was found that the raising the pH of the
solution affecting on the rate of oxide film formation and the breakdown
potential value.
29
Introduction The inhibiting effect of azithromycin toward the corrosion of zinc in
various concentrations ( 0.01 to 0.05 M ) of H2SO4 was studied using weight
loss and hydrogen evolution methods of monitoring corrosion [65] .The results
revealed that various concentrations of azithromycin (0.0001 to 0.0005 M)
inhibited the corrosion of zinc in H2SO4 at different temperatures (303 to 333
K). The concentration of H2SO4 did not exert significant impact on the inhibition
efficiency of azithromycin, but inhibition efficiencies were found to decrease
with increase in the concentration of the inhibitor. Values of inhibition
efficiency obtained from the weight loss measurements correlated strongly with
those obtained from the hydrogen evolution measurements. The activation
energies for the corrosion of zinc inhibited by azithromycin were higher than the
values obtained for the blank. Thermodynamic data revealed that the adsorption
of azithromycin on the surface of zinc was endothermic, spontaneous and was
consistent with the adsorption model of Langmuir.
Chromates conversion coatings provide very effective corrosion
protection for many metals. However, the high toxicity of chromate leads to an
increasing interest in using non-toxic alternatives such as molybdates, silicates,
rare earth metal ions and etc. In this work, quartz crystal microbalance (QCM)
was applied as an in-situ technique to follow the film formation process on zinc
(plated on gold) in acidic solutions containing an inorganic inhibitors, e.g.
potassium chromate, sodium silicate, sodium molybdate or cerium nitrate [66].
Using an equation derived in this work, the interfacial mass change during the
film formation process under different conditions was calculated, indicating
three different film formation mechanisms. In the presence of K2CrO4 or
Na2SiO3, the film growth follows a mix-parabolic law, showing a process
controlled by both ion diffusion and surface reaction. The apparent kinetic
equations are 0.4t = 17.4 + 20 mf + ( mf)2 and0.1t = 19.0 + 8.4 mf
+ 10( mf)2 respectively (t and m are in seconds and g/cm2). In solutions
30
Introductioncontaining Na2MoO4, a logarithmic law of m f = 24.7 + 6.6 in t was
observed. Changing the inhibitor to Ce(NO3)3, the film growth was found to
obey an asymptote law that could be fit into the equation of
mf = 55.1(1 exp( 2.6 × 10 3t)).
The inhibiting effect of cetylpyridinium bromide (CPB) toward
corrosion of zinc in hydrochloric acid was investigated using weight loss
method [67]. Experiments show that the adsorption of CPB on zinc is the key to
inhibition, and the Langmuir isotherm is followed. Some important
thermodynamic parameters in adsorption process were calculated from testing
data by Sekine method.
In order to study the alkyl benzene sulfonate anion surfactant's
corrosion inhibition and adsorption for the zinc in nitric acid, the corrosion
inhibition of sodium do decylbenzene sulfonate on zinc in 3% nitric acid was
Investigated with different temperatures and different concentration of sodium
do decylbenzene sulfonate by weight-loss method [68]. Results showed that
sodium do decylbenzene sulfonate had efficiently inhibited the corrosion of zinc
.After its content achieved the certain concentration, the corrosion inhibition is
in gross invariable. It was found that the adsorption of sodium do decylbenzene
sulfonate on the surface of zinc is the important reason resulted from corrosion
inhibition and that the rule of adsorption conforms to Langmuir’s isotherm in
0~300 mg/L concentration. The experimental data were treated with Sekine
method. Some thermodynamic parameters ,such as H0 S0 and G0 are
obtained.
Quantum chemical methods are particularly significant in the study of
electrochemistry and provide researchers with a relatively quick way of studying
the structure and behavior of corrosion inhibitors [69]. The originality of this
review article is based on the fact that it is the first and unique general reference
31
Introductionfor all those interested in the use of quantum chemical methods in corrosion
inhibitor studies.
The inhibition effect of ethoxylated fatty acids were used as inhibitors
for the corrosion of zinc metal in 1.0 M hydrochloric and 1.0 M sulfuric acid
solutions at various temperatures ranging from 25 to 55°C was investigated by
weight loss and electrochemical methods [70].The protection efficiency depends
upon the type and concentration of the inhibitor and the nature of the acid
medium. In both acid solutions the protection efficiencies of the inhibitors
decrease with the increase in temperature. The inhibition was assumed to occur
via the adsorption of the fatty acid molecules on the metal surface. The
thermodynamic functions of dissolution and adsorption processes were
calculated and discussed.
The passivation and pitting corrosion behavior of a zinc electrode in
aerated neutral sodium nitrate solutions was investigated by cyclic voltammetry
and chronopotentiometry techniques, complemented by exsitu scanning electron
microscopy (SEM), X-ray diffraction (XRD) and energy dispersive X-ray
(EDX) examinations of the electrode surface [71]. The potentiodynamic anodic
polarization curves do not exhibit active dissolution region due to spontaneous
passivation. The passivity is due to the presence of thin film of ZnO on the
anode surface. The passive region is followed by pitting corrosion as a result of
breakdown of the passive film. SEM images confirmed the existence of pits on
the electrode surface. The breakdown potential decreased with an increase in
NO3 concentration and temperature, but increased with increasing potential
scan rate. Addition of SO4 ions to the nitrate solution accelerates pitting
corrosion, while addition of WO4 and MoO4 ions inhibits pitting corrosion.
The chronopotentiometry measurements show that the incubation time for
pitting initiation decreased with increase of NO3 concentration, temperature
32
Introductionand applied anodic current density. Addition of SO4 ions decreased the rate of
passive film growth and the incubation time, while the reverse changes produced
by addition of either WO4 or MoO4 ions.
The inhibition of corrosion of zinc in 1.5M nitric acid, by some mono
saccharides (glucose, fructose, mannose and galactose) in the concentration rang
5×10-2 to 3×10-1 M had been studied with respect to concentration of inhibitor
and period of immersion [72]. In general, the inhibitor efficiency increased with
the increase of the concentration of inhibitor due to the adsorption of these
compounds on the zinc surface.
Tertiary arsines such as diphenylethylarsine (DPEA), diphenylmethyl
arsine (DPMA), diphenyl arsine (TPA) have been used as Zn inhibitors for
corrosion of zinc in perchloric acid solution [73]. These inhibitors inhibit
corrosion effectively even in traces. The percentage inhibitor efficiency of the
inhibitors are in the order DPEA > DPMA > TPA. The effect of pH and
temperature on the inhibitor efficiency is studied. Steady state anodic and
cathodic polarization data reveal that tertiary arsines act as interfacial corrosion
inhibitors for zinc in acidic solution. The apparent free energies of adsorption of
inhibitors have been reported for different possible modes of adsorption.
The corrosion of zinc in aqueous methanolic solution containing tri
chloroacetic acid was studied [74]. Results indicated that the rate determining
step in the corrosion was likely to be diffusion and /or adsorption of the
participating species. The corrosion process of Zn was partly inhibited by the
blockage of the surface by the adsorbed MeOH molecules.
The initial stage of atmospheric zinc corrosion using exsitu
electrochemical impedance spectroscopy (EIS) in methanol electrolyte [75].
Compared with the traditional techniques for studying atmospheric corrosion,
33
Introductionsuch as gravimetry, the EIS technique significantly reduced the exposure time
for detectable corrosion at any relative humidity from several days to a few
hours. The samples were first exposed to synthetic atmospheres with careful
control of O2 and CO2 concentrations, relative humidity and temperature. EIS
was then used to measure the polarization resistance (Rp) of the exposed
samples. The corrosion products were analyzed by a combination of grazing-
angle X-ray diffraction, Fourier transform infrared spectroscopy and
photoelectron spectroscopy. Several interesting phenomena occurring in the
initial stage of corrosion were demonstrated by studying the electrochemical
properties of the surface layer formed on the zinc. At high values of relative
humidity (RH 95-100%) with CO2 >40 ppm, the Rp of the surface film formed
increased monotonically with time and relative humidity. At intermediate values
of relative humidity ( RH 50-85% ) in the presence CO2 ( 40-500ppm ), Rp first
increased with time, reached a maximum, and then fell from the maximum value
before again rising slowly.
The inhibition of zinc corrosion by various triazole derivatives was
investigated with respect to dependence on pH, potential E, inhibitor
concentration Cinh., zinc ion concentration Czn2+ and exposure time t. current-
Potential curves and measurements of weight loss showed that 3-amino-5-
heptyl-l,2,4-triazole (AHT) is an excellent inhibitor of zinc corrosion in acid and
weak alkaline solutions, while bisaminotriazole (BAT4 gives the best inhibition
efficiency especially at pH 9 and low inhibitor concentration) [76] .Capacity
potential curves and XPS-measurements show the formation of protective
triazole layers with a maximum thickness of about 3nm. The chemical analysis
of insoluble zinc triazole complexes precipitated in solution yields a
stoichiometric ratio of Zn2+ and the triazole ring of 1:2. The presence of zinc
triazole complex on the zinc surface is shown by XPS spectroscopy. The
solubility, adsorbability and hydrophobicity of the triazole molecules and the
34
Introductionstability of the zinc-triazole complexes are found to be the most important
factors influencing the inhibition efficiency of the triazole derivatives under
varying condition.
The effect of quinoline, benzo (f) quinoline and 8-hydroxyquinoline on
the electrochemical and corrosion behavior of zinc, Zn 2% Cd and Zn 2% Pb
alloys in deaerated 0.1 M HCl solution was studied using the potentiostatic
technique [77]. Although quinoline inhibited the corrosion of zinc at all
examined concentrations, it accelerated the corrosion of the alloys. As an
inhibitor, quinoline was found to have a predominant anodic effect and its
adsorption conformed to the Temkin isotherm at concentrations >10-4M, benzo
(f) quinoline inhibited the corrosion of both zinc and the alloys. Inhibition was
found to be predominantly anodic without changing the mechanism of zinc
dissolution, inhibition by 8-hydroxyquinoline was found to be purely anodic
occurring by surface chelation, resulting in a change of the mechanism of zinc
dissolution.
The corrosion behavior of zinc in stagnant distilled water containing 0-
70 percent (v/v) methanol, ethanol or n-propanol was investigated at 25 - 40 C
using potentiodynamic polarization technique [78]. The activation parameters
that govern zinc corrosion in mixed solvent system were also calculated. The
data revealed that, the corrosion of zinc in mixed solvents depends on two
factors : the hydrolysis rates of the metals ions in alcohol-water solutions and
the chemisorption of organic solvent molecules at the metal surface .When the
later effect predominant the final result is an increase of the inhibiting effect. On
the other hand, when the first factor is dominant the final result is a decrease in
the protection efficiency and may exhibit an accelerating effect. Special
attention should be made on using mixed water-alcohol solvents, 50 percent
(v/v) has unexpected accelerating effect. Whereas 70 percent ( v/v ) exhibits
35
Introductionprotection efficiency of 58 percent. Owing environmental concerns, the use
of alcohol in automotive fuel increases. Therefore, it is of importance to study
the corrosion behavior of zinc in alcoholic solution.
Rangel et al [79] studied the corrosion and inhibition corrosion of zinc
by polyphosphate, using electrochemical impedance spectroscopy (EIS) and
cyclic voltammetry artificial waters as electrolytes. The anodic dissolution
reaction inhibited mixed kinetics, which changed to diffusion control when
calcium ions were removed. The effect of calcium to polyphosphate ratio on the
corrosion rate of zinc was studied to determine the composition, which gave
the optimum protection.
The corrosion and electrochemical properties of three zinc single
crystal surfaces with different orientations had been investigated [80]. In near-
neutral 1 M ( NH4 )2SO4, the corrosion rates on all three surfaces were found to
be similar. However, the SEM morphologies of the corresponding corroded
surfaces were markedly different from each other, but consistent with
preferential attack of {11 0} surfaces. In alkaline 0.5 N NaOH solution, the
three sample orientations showed significantly different reactivities, with the
(11 0) surface exhibiting the highest reactivity and corrosion rate. Here,
passivation ultimately occurred and the difference in the corrosion performance
of the three surfaces, even for small over voltages, is attributed to the presence
of oxide or hydrated oxide films.
Quantum mechanical calculations have been applied to a series of
pyrazole derivatives used as corrosion inhibitors for zinc, copper and - brass in
order to assess quantum chemistry as means of evaluating effectiveness of
corrosion inhibitors [81]. The corresponding structures have been optimized
the energies and coefficients of their molecular orbitals (HOMO and LUMO )
have been computed using the semi empirical method, MNDO. The theoretical
36
Introductionresults are then compared with experimental data. The inhibition action occurs
via chelation of the active centres with the metal surface. The position of the
substituent group with respect to the pyrazole ring also affects the inhibition
efficiency of the inhibitor molecules [82].
The influence of benzotriazole (BTA) on the corrosion of pure zinc in
1NH2SO4 has been studied by weight loss method[83]. The additive
concentrations in the range of 10-5M to 10-1M were examined at four different
temperatures. The heat of adsorption and the activation energy were calculated
for the corrosion inhibition process. Finally, free energy change for the
corrosion inhibition process had been found out. It was, observed that
benzotriazole acted as a good inhibitor for the corrosion of zinc at low
temperatures. At higher temperatures the inhibition efficiency decreased
considerably.
The corrosion of zinc in ammonium chloride, both pure and containing
various organic compounds, has been studied by means of analytical and
electrochemical methods as determination of the zinc dissolved and the
hydrogen evolved, I-V curve recording and impedance measurements [84]. The
obtained results in a potential region near the zinc corrosion potential showed
that the cathodic reaction of hydrogen discharge does not fit a simple
exponential law because the Tafel coefficient appears to be electrode potential
dependent. The experimental findings proved that only five of the 25 organic
substances tested were found to behave as zinc corrosion inhibitors in
decreasing order of influence.
Impedance diagrams for protected and non-protected zinc were recorded
in chloride solutions using stationary and rotating disk electrodes [85]. Some of
the determined impedance parameters can be very useful in the characterization
of the processes of corrosion protection and diffusion.
37
Introduction Pitting corrosion of zinc in neutral inhibited conditions was studied
[86]. The effect on addition of aggressive salts such as LiCl, NaCl, RbCl and
MgCl2 on the steady-state potential of zinc electrode previously equilibrated in
passivating chromate solution was established. S-shaped curves were obtained
for the variation of the steady state potential with good environmental
characteristics. The quantity of aggressive salt added for each inhibitor
concentration, the addition of aggressive ions up to a certain concentration has
no effect on the passivity of zinc. However, Cl- ion concentration caused
destruction of the passive film and initiation of pitting corrosion. Destruction of
passivity occurred after an induction period, which decreases with the increase
in the concentration of the attacking ion or the decrease in that of the inhibiting
ions. The efficiency of these salts in initiating pitting corrosion increased in
the order: RbCl < MgCl2< KCl < NaCl < LiCl . The change in the degree of
aggressively of these salts could be attributed either to the incorporation of the
cations in the passive film or to their effect on pH.
The kinetic of corrosion of pure zinc in trichloracetic (TCA) was
investigated over a wide range of solvent compositions and temperatures
[87] . The variation of thermodynarnic properties of the activated complex
with the mole fraction of methanol revealed the existence of preferential
solvation. The determined isokinetic temperature indicated that the reaction was
enthalpy controlled where the interaction between methanol and zinc surface
played an important role.
The electrochemical behavior of zinc in NaOH solutions was
investigated by using potentiodynamic technique and complemented by X-ray
analysis [88]. The E/I curve exhibit active, passive and transpassive regions
prior to oxygen evolution. The active region displays two anodic peaks. The
passivity due to the formation of a compact Zn(OH)2 film on the anode
38
Introductionsurface. The transpassive region is assigned to the electro formation of ZnO. The
reverse sweep shows an activation anodic peak and one cathodic peak prior to
hydrogen evolution. The influence of increasing additives of NaCl, NaBr and
NaI on the anodic behavior of zinc in NaOH solutions has been studied. The
halides stimulate the active dissolution of zinc and tend to break down the
passive film, leading to pitting corrosion. The aggressiveness of the halide
anions towards the stability of the passive film decreases in the order: I- >Br-
>CI-. The susceptibility of zinc anode to pitting corrosion enhances with
increasing the halide ion concentration but decreases with increasing both the
alkali concentration and the sweep rate.
Inhibiting effect of semicarbazide, thiosemicarbzide,sym
diphenylcarbazid towards corrosion of zinc in hydrochloric acid had been
investigated [89]. The rate of corrosion depended on the nature of the inhibitor
and its concentration. The values of inhibition efficiency from, weight-loss
thermometric measurements were in good agreement with those obtained from
polarization studies. The inhibitors used acted as mixed adsorption type
inhibitors Increased adsorb resulting from an increase in the electron density at
the reactive C=S, C=O groups and N-atoms. The thermodynamic parameters of
adsorption obtained using Bockris-Swinkels adsorption isotherm revealed a
strong interaction of these carbazide on zinc surface.
The inhibiting effect of p-chlorobenzylidencyanothioacetamide towards
corrosion of zinc in 1N HCl using three different techniques e.g.( weight loss,
volume of hydrogen evolution and polarization measurements) [90]. All the
results of the three methods were in good agreement that the inhibitor efficiency
increased with increase of inhibitor concentrations. The degree of surface
coverage varied linearly with logarithm of inhibitor concentration fitting Temkin
isotherm. The inhibitor molecules were physically adsorbed on the zinc surface
39
Introductionas indicated from the decrease in the inhibitor efficiency with the rise of
temperature.
The corrosion behavior of electrodeposited zinc and Zn-Ni alloy in
synthetic sea water with and without sulphide present as a pollutant was studied
[91]. Zinc undergoes galvanic dissolution giving rise the corrosion products
consisting mainly of zinc oxide and hydroxide. Unless these products form a
compact layer, atmospheric oxygen will have easy access to the steel surface
and the corrosion of zinc will be accelerated.
The anodic behavior of Zn in O.1 M NaOH containing various
concentrations of Na2SO4, Na2SO3, Na2S, Na2S2O3 or NH4SCN was studied by
means of the potentiodyamic technique, complemented by X-ray diffraction
analysis and scanning electron microscopy [92]. In the absence of sulphur-
containing anions in solution, the cyclic voltammogram displays two anodic
peaks in the forward scan prior to reaching the oxygen evolution potential. The
first anodic peak A1 is related to the electro formation of Zn(OH)2, while the
more positive peak A2 is assigned to the formation of ZnO2.The reverse scan
exhibits a reactivated anodic peak A3 and one cathodic peak C1, prior to
reaching the hydrogen evolution potential. The presence of either S042- or SO3
2-
Stimulates the active dissolution of Zn while the presence of S2- ( and / or SH-
), S2O32- or SCN- inhibits it, presumably as a result of electro formation of
sulphur-containing solid phases preceding the formation of Zn(OH)2. Also, the
presence of one of the cited anions studied in the alkali solution produces
pitting of Zn at a certain specific pitting potential. The existence of pitting
is confirmed by scanning electron microscopy. The aggressiveness of the
sulphur species decreases in the order SCN-> S042- > SO3
2- > S 2O32-> S2- .The
pitting potential decreases with increasing concentration of the sulphur species.
40
Introduction The corrosion behavior of Zn and ZnO coatings in 3.5 % NaCl
solution, with and without illumination, was investigated by weight loss and
potentiodynamic anodic polarization measurements [93]. The ZnO films, of a
range of thicknesses, were formed by anodizing in borate solution. In dark
conditions, the corrosion rate was found to increase initially with increase in the
oxide thickness, whereas above a certain thickness the corrosion rate
decreased. These results are attributed to a thin layer of oxide with a non-
uniform structure formed during times of 1–4 hr. anodizing, while for longer
times of anodizing, a thicker oxide is formed. The corrosion products consist
mainly of Zn (OH)2. Under illumination conditions, the corrosion rate
decreased as the oxide thickness increased, due to an initial, more rapid
formation of corrosion products than in the dark, which creates a protective
layer, delaying further corrosion. The corrosion products consist mainly of
zinc oxy chlorides, which have more protective properties than the Zn (OH)2.
The presence of zinc oxychlorides in the corrosion products increases with
time of anodizing. For thin films, corrosion rates were greater under
illumination, but for thicker films, corrosion rates were greater in the dark.
Uniform corrosion was observed in dark conditions, but under illumination,
pitting corrosion occurred and higher current densities were observed than in the
dark.
The inhibition corrosion processes of zinc using furfural as chemical
inhibitor in ethanolic solutions was studied [94]. The results obtained from
different electrochemical methods confirmed this effect that, inhibition
depended upon the adsorption step and was affected by the incidence of
polychromatic light on the metal surface. Protection efficiency of furfural was
higher when the system was kept in the dark than when under illumination, An
inhibition process involving and adsorption step of furfural was proposed.
41
Introduction Mlieller et al [95, 96] investigated the inhibition corrosion of zinc
pigments in an aqueous alkaline paint medium by hydrogen evolution. The
corrosion reaction was inhibited by addition of chelate form aromatic-
hydroxyl compounds of low molecular weight. Propoctly, dodecygallate and
low molecular weight. Styrene maliec acid and styrene-acrylic acid/styrene-
acrylate copolymers were found to be good inhibitors for zinc pigments.
Inhibition by oligomeric phenolic (resole) could be explained by a chelation
reaction with zinc (II).
Zinc pigments react in aqueous alkaline media (e.g. water-borne
paints) by the evolution of hydrogen. Heterocyclic compounds can inhibit
this corrosion reaction [97]. The corrosion inhibiting effect of the heterocyclic
depends strongly on the base which is used for adjusting the pH value to 10.
Ammonia accelerates the corrosion reaction very much; the protection factors
for the heterocyclic are 5–40%. The examined heterocyclic inhibit significantly
better than dimethyl ethanolamine or sodium hydroxide are used as bases,
protection factors increase up to 98%.
Zinc pigments react in aqueous alkaline media (e.g. water-borne
paints) by the evolution of hydrogen [98]. This corrosion reaction can be
inhibited at a pH value of 10 (DMEA) by the addition of amino methylene
phosphonic acids, amphiphilic partial esters of phosphoric acid and phosphoric
acid. The protection factors for the corrosion inhibition of zinc pigment (
borderline Lewis acid ) by salts of amino methylenephosphonic acids and
phosphoric acid ( hard Lewis bases ) are smaller for the corrosion inhibition of
aluminum pigments ( hard Lewis acid ).
Zinc pigments react in aqueous alkaline media (e.g. water-borne paints)
by the evolution of hydrogen which can be measured gas volumetrically [99].
The addition of both pure citric acid and pure metal salts stimulated the
42
Introductioncorrosion reaction of zinc pigment in aqueous alkaline media. The addition of
soluble zinc (II) and aluminum(III) chelates of citric acid reduced the
hydrogen evolution of zinc pigment dispersions compared to pure citric acid;
but the level of hydrogen evolved was still very high. In contrast, soluble
cerium(III) chelates of citric acid were powerful corrosion inhibitors for zinc
pigments in aqueous alkaline media. The pronounced corrosion inhibiting
effect of the cerium(III) chelates could be attributed to cerium and not to the
charge of the chelates.
The rate of corrosion of electroplated zinc in near-neutral chloride
solutions can be lowered by as much as 75% by adding fine, inert
particles of substances such as MnO2, Fe3O4, SiC and TiN to the well-stirred
solution [100]. Spreading of local areas of etching is also stopped.
Corrosion studied showed normal dodecylamine (NDDA) to be
effective as anodic inhibitor for zinc corrosion in ammonium chloride media
[101]. The inhibiting effect was found to be associated with adsorption on
active centers of the zinc surface. The adsorption was potential dependent
and the inhibiting effect diminished with positive shift of the zinc electrode
during discharge. This feature makes it possible for NDDA to be used in
zinc-manganese dry batteries as a substitute for the environmentally
harmful mercury inhibitor.
Pourbaix diagrams ( potential / pH diagrams ) for zinc at 25–300°C
was revised [102]. The diagrams were calculated for three concentrations,
10-5, 10-6 and 10-8 ml-1, the latter for use in high purity water such as in
nuclear power reactors. Extrapolation of the electrochemical data to elevated
temperatures was performed with the revised model of Helgeson-Kirkham-
Flowers, which also allows uncharged aqueous complexes, such as
43
IntroductionZn(OH)2(aq), to be handled. The calculations show that the hydroxide of zinc
does not passivate at the concentration of 10-6 M, due to the uncharged zinc
(II)complex, Zn(OH)2(aq).However, at concentrations 10-5.7 ml-1 zinc
passivates by formation of ZnO, which is stable in the temperature interval
investigated.
The corrosion of Zn in both aerated by air (O2 + N2), and deaerated by
N2, solutions of KNO3 at concentrations largely varying, 10-5 to 1 M, was
studied [103]. The corrosion process is followed in time by potentiometry and
the potential of Zn electrode (vs. SCE) vs. time plots is derived. In both cases, a
transition period is observed until a steady state is achieved where the rate
of Zn2+ and OH- production becomes equal to the rate of Zn(OH)2 precipitate
formation. In the aerated solutions the potential and corrosion rate decrease
with time while in the de-aerated solutions they pass successively through a
minimum and a maximum before a steady state is achieved. By a suitable
potentiometric analysis the results are explained. The most important factors
found to be affecting the mechanism of corrosion process are presented. From
the discovery of the mechanism and the factors affecting the Zn corrosion,
predictions for promoting or slowing down the corrosion may be derived.
The corrosion behavior of zinc and its alloys in H2SO4 solutions with
oxygen and ferric ions was studied using a potentiostatic polarization[104]
.Cathodic reduction of oxygen mainly was controlled by chemical reaction
and that of Fe3+ ions was controlled by diffusion. The overall cathodic
process was the summation of the reduction of oxygen and Fe3+ ions,
corrosion of zinc controlled mainly by reduction of water. Benzotriazole
(BTAH) was used as corrosion inhibitor for dissolution of zinc in 1M
H2SO4 investigated in aerated and deaerated solutions. BTAH was found to
be a useful inhibitor, and the inhibition layer was shown to be stable and
44
Introductionpersistent. By scanning electron microscopy (SEM) the surface of zinc after
corrosion in the presence and absence of BTAH was examined that the
inhibiting effect of BTAH towards the corrosion of zinc is due to
formation of a protective film.
The inhibiting properties of some organic phosphonium and ammonium
compounds were studied with respect to the corrosion of zinc in 1M H3PO4
solution [105] . It was found that onium compounds which have -electron
system are adsorbed following Frumkin's adsorption isotherm and provide
better inhibition efficiency than those containing no electron system. The
adsorption of the latter compounds was found to obey Langmuir's isotherm.
Both potentiostatic and electrochemical impedance techniques proved that
the studied onium compounds act as primary interface inhibitors without
changing the mechanism of either hydrogen evolution reaction or zinc
dissolution.
The corrosion behavior of zinc metal in some organic solvents was
tested electrochemically using galvanometric polarization measurements
[106].Results showed that the studied organic solvents act as mixed type
inhibitors. The inhibition was assumed to occur via physical adsorption of
the inhibitor molecules, fitting a Temkin's isotherm. The inhibition efficiency
of the solvents increase in the order: glycerol > ethyleneglycol >DMSO
>dioxane. This order is not affected by the variation in temperature in the range
35-55°C. The increase in temperature was found to increase the corrosion in
absence and presence of inhibitors. Some thermodynamic parameters for
adsorption were also computed and discussed.
The effect of a new organic molecule with chelating groups on the
corrosion behavior of zinc was investigated [107]. Electrochemical studies
of the zinc specimens were performed in aerated aqueous solution (0.2M
45
IntroductionNa2SO4 + 0.2M NaCl, pH 6) by means of electrochemical impedance
spectroscopy ( EIS ), as well as polarization curves using a rotating disk
electrode. The treatment was achieved by immersion in solutions of
variable concentration of the organic molecule and for different immersion
times. The influence of the treatment bath temperature was also studied.
The recorded electrochemical data showed that the corrosion resistance of
the treated samples was greatly enhanced when compared to the untreated
zinc. Moreover, the treatment induced a basic modification of the cathodic
corrosion behavior of zinc decreasing the electron transfer rate. The corrosion
inhibition could be explained by a chelation reaction between zinc and
organic molecule and the formation of a protective organometallic layer on
the metal surface. Fourier Transform Infrared Spectroscopy ( FT-IR ) was
applied in order to investigate this layer.
The anodic behavior of zinc in Na2B4O7 solutions in the absence and
presence of Cl- anion as an aggressive anion has been investigated by the
galvanostatic polarization technique [108]. In the absence of Cl- anion, the
polarization curves are characterized by one distinct arrest corresponding to
Zn(OH)2 or ZnO after which the potential increases linearly with time
before reaching the oxygen evolution region. Addition of low concentrations
of Cl- anions, has no effect on the passive film formed on the metal
surface. The potential starts to oscillate within the oxygen evolution region
with an increase in the concentration of the Cl- anion. This suggests
interference of Cl- anion with oxygen evolution. Further increases in the
concentration of Cl- anions are associated with breakdown of Zn passivity, this
denoting the destruction of the passivating film and initiation of pits. The
susceptibility of zinc to pitting corrosion is enhanced with increasing Cl- anion
concentration but decreases with an increase of both the Na2B407 concentration
and current density. Addition of increasing concentrations of tungstate,
46
Introductionphosphate or molybdate anions causes a shift of the breakdown potential
in the noble direction, indicating the inhibitive effect of the added anion on the
pitting attack. In contrast, addition of the sulphate anion enhances the pitting
attack.
The pitting corrosion behavior of Zn in neutral ( pH 6.8 ) Na2SO4
solutions was studied by using potentiodynamic and cyclic voltammetry
techniques and complemented by X-ray analysis under the effect of electrolyte
concentration, scan rate, temperature and pH[109]. The voltammograms involve
active/passive transition prior to the initiation of pitting corrosion. The active
region displays one anodic peak. The passivity is due to the formation of
ZnO film on the anode surface. The critical pitting potential decreases with
increasing sulphate ion concentration and temperature but decreases with
scan rate. Increasing the acidity or alkalinity of the medium enhances the
pitting corrosion. The effects of adding increasing concentrations of Cr2O42-,
Cr2O72-, WO4
-2, MoO42- and NO2
- anions on sulphate pitting corrosion of Zn
were investigated.These anions inhibit the active dissolution and pitting
corrosion and the extent of inhibition depends upon the type and concentration
of the inhibitors. The adsorption characteristics of these anions on the electrode
surface play a significant role in inhibition.
Using a simple model cell the susceptibility of the zinc electrode
to pitting corrosion by SO42-, SO3
2-, S2O32- and S2- anions were examined in
naturally aerated carbonate solutions[110]. It was found that, pitting started
after an induction period , which depended on the type and concentration
of the aggressive and passivating anions. The pitting corrosion current increased
with time until steady state values were attained. These values depended on
both the type and the concentration of the passivating and pitting anions. For
the same concentration of the passivating anions, the corrosion current varied
47
Introductionwith the concentration of the aggressive anion according to the relation:
logipit.=a1+b1logCagg. . At a constant concentration of the aggressive anion, the
corrosion current varied with the concentration of the passivating anions
according to: logipit.=a2-b2logCpass. The constants a1 (a2) and b1 (b2) were
determined for all the systems studied. From the values of a1 the
corrosivity of the sulphur-containing anions is found to decrease in the
order SO42->SO3
2->S2O32->S2-.
The corrosion behavior of zinc deposits obtained under pulsed
current electrodeposition from an acidic chloride bath in the presence and
absence of coumarin has been investigated [111]. The effects of pulse peak
current density (Jp) on the morphology of zinc deposits were studied by
scanning electron microscopy. An increase in Jp from 40 to 280 A dm 2 yields
deposits with a finer grain size. The refinement of the grain size was more
considerable in the presence of coumarin ( Jp = 280 A dm 2 ). The preferred
orientation of zinc deposits was studied by X-ray diffraction. At Jp
= 40 A dm 2, the preferred orientation of zinc deposits was (1 0 3) and changed
to (0 0 2) at Jp = 80 A dm 2. An increase in Jp to 280 A dm 2 did not change the
preferred crystallographic orientations except for an increase in the peak
intensity of the (0 0 2) plane. In the presence of coumarin, the preferred
crystallographic orientations changed at Jp = 280 A dm 2 from the (0 0 2) plane
to the (1 0 3) plane. The corrosion behavior was investigated in an aerated
3.5% NaCl solution; the anodic polarization and electrochemical impedance
spectroscopy curves were performed. The corrosion resistance of zinc deposits
was improved by increasing the pulse peak current density (Jp); whereas, the
presence of coumarin did not improve the corrosion resistance.
The performance of ethylene diamine, N, N`-dibenzylidene, ethylene
diamine,N,N`-di (p-methoxybenzylid-ene),ethylenediamine N,N`- disalicylidene
48
Introductionwere used as corrosion inhibitors for zinc in sulphuric acid [112].The effect of
various parameters on the efficiency of these inhibitors had been studied.
Ethylene diamine, N, N`-di (p- methoxy benzylidene) and ethylene diamine N,
N`-di salicylidene give 99% protection under a variety of conditions. Activation
energies in the presence and absence of inhibitors have been calculated. It
appears that an efficient inhibitor is characterized by a relatively greater
decrease in free energy of adsorption, relatively lower entropy of adsorption
and relatively lower heat of adsorption. Galvanostatic polarization studies
indicate that these are basically cathodic inhibitors. Cathodic protection in the
presence of these inhibitors was studied. With an efficient inhibitor, cathodic
protection was achieved at potentials much less negative than that required for
plain acid. The difference between protective potential and corrosion potential
appears to be less for an effective.
The inhibiting effect of 2-mercaptobenzothiazole(MBT) self-
assembled monolayer on silver and zinc electrodes were comparatively studied
[113] by means of electrochemical impedance spectroscopy ( EIS ). The
adsorption geometries of MBT monolayer on zinc and silver electrodes
were observed by surface enhanced Raman scattering ( SERS ) technique .
The SERS spectra implied that monolayer of MBT could be self-assembled
on Ag surface through S10 and N3 atoms and the molecular plane should
be tilted with respect to the surface . On Zn surface, MBT molecules formed
monolayer via both S atoms the other moieties of the molecule away from the
surface. From the insitu electrochemical SERS results it can be found that MBT
monolayer on both Ag and Zn surfaces experienced the changes of adsorption
fashions as the potential shifting to more negative direction.
The effect on the corrosion behavior of zinc of a new organic molecule
with chelating groups was investigated [114]. Electrochemical studies of the
49
Introductionzinc specimens were performed in aqueous sulfate–chloride solution (0.2 M
Na2SO4 + 0.2 M NaCl, pH 5.6 ) using potentiostatic polarization techniques
with a rotating disk electrode. Zinc samples, previously treated by immersion
in the inhibiting organic solution, presented good corrosion resistance . The
in uence of the treatment bath pH and temperature on the protection efficiency
has been emphasized. The recorded electrochemical data indicated a basic
modi cation of the cathodic corrosion behavior of the treated zinc resulting in
a decrease of the electron transferrate. Corrosion protection could be
explained by a chelation react ion between zinc and organic molecules and the
consequent growth of an organometallic layer strongly attached to the metal
surface which prevented the formation of porous corrosion products in the
chloride-sulfate medium. This protective lm was studied using several
surface analysis techniques such as X-ray photoelectron spectroscopy (XPS),
scanning electron microscopy ( SEM–EDS ) and Fourtier transform infrared
spectroscopy ( FTIR ).
The corrosive behavior of zinc in HCl solution containing various
concentrations of glutaraldehyde GTD, glycine GLN, methionine MTN, and
their condensation products formed between GTD + GLN (CP1) and GTD +
MTN (CP2) was investigated [115]. The corrosion-inhibitive action of these
compounds on zinc metal was studied using chemical and electrochemical
methods. The results showed that the compound CP2 is the best inhibitor
and that its inhibition efficiency reaches 92.56 % at 10 2 M in 0.05 M acid
concentration. As an inhibitor, CP2 was found to have a predominant cathodic
effect and its adsorption was confirmed with the Temkin isotherm. The
effect of temperature on the corrosion of zinc was investigated by the
weight-loss method. The morphology of the corroded surface was studied by
SEM technique to obtain information about the adsorption of inhibitor
molecules on the zinc surface.
50
Introduction Sulphur containing organic compounds decreased the corrosion
rate by increasing the hydrogen over potential on zinc metal due to their
electron donating groups. Their inhibiting effect was found to be associated
with their adsorption on the active centers of the metal. The inhibition
efficiencies of some aliphatic sulphide in ammonium chloride solution have
been studied by weight loss studies, polarization and impedance measurements
[116]. The effect of substituent groups is correlated with their inhibition
performance. These studies due to their relevance in Zn Manganese dry
batteries assume their importance.
The reaction of water based solution of 1,5-diphosphonopentane (
DPP ) with high purity polycrystalline zinc surface was investigated at
room temperature [117] . XRD and XPS studies evidenced the formation of
crystalline zinc –phosphonate film on the metal surface. The former layers
give hydrophilic properties to the surface. A conversion-type interaction of
diphosphonic acid compounds with the oxidized zinc surface was
unambiguously shown by XPS analysis. Conclusive results were obtained
by synthesized zinc-diphosphonate model compounds imitating the ones
developed on the zinc surface, revealing a simple, closely 1:1 ratio of the
phosphonate groups with zinc . Reactions with both diphosphonates resulted
in significant protective effect of zinc against corrosion, although the
structure and quality of the formed layers exhibit marked differences. The
in-depth distribution of the composition and dissimilarity of the layer
thickness were determined by glow discharge optical emission spectrometry.
The corrosion inhibition was explained by the formation of insoluble zinc-
phosphonate salt on the zinc surface, blocking the zinc dissolution process.
Quantum chemical methods are particularly significant in the study
of electrochemistry and provide researchers with a relatively quick way of
51
Introductionstudying the structure and behavior of corrosion inhibitors[118]. The originality
of this review is based on the fact that it is the first and unique general
reference for all those interested in the use of quantum chemical methods in
corrosion inhibitor studies. It begins with a concise summary of the most
used quantum chemical parameters and methods.
The efficiency of various imidazole derivatives as zinc corrosion
inhibitors in hydrochloric acid ( HCl ) was investigated using electrochemical
and gravimetric methods[119] . A difference in efficiency (z) was observed
with the introduction of different substituents into an imidazole skeleton. It
was established that substituents which increased the electron density at the
reaction center improved the corrosion inhibiting properties of the heterocyclic
compound. A linear correlation between reaction kinetic data and the sum of
the polar substituent constants was obtained.
The corrosion inhibition of zinc in 0.1M HCl in presence and
absence of some hydrazide derivatives was investigated using mass-loss
and polarization techniques[120].The obtained results showed that the
inhibition efficiency increased with the increase of the concentration of the
additives and decreased with the increase of temperature. Synergism
between I- , SCN- and Br- anions and hydrazide derivatives was proposed.
The polarization curves showed that hydrazide derivatives act as mixed-
type inhibitors, acting predominantly as cathodic inhibitors for zinc in 0.1
M HCl .The adsorption of these hydrazide derivatives on zinc surface
follows Temkin’s adsorption isotherm. Some thermodynamic parameters were
calculated. The kinetic parameters of corrosion of zinc in HCl solution have
been studied.
[[
52
Introduction Adsorption of the quaternary ammonium Gemini surfactants
alkanediyl- -bis-( dimethyl dodecyl ammonium bromide ), referred as C12-s-
C12·2Br (s = 2,3,4,6), on the zinc surface in 0.5 mol/L H2SO4 solution was
investigated using gravimetric measurements [121]. The adsorption of C12-s-
C12·2Br on the zinc surface accorded to the Frumkin isotherm model. The
maximal inhibition efficiency for zinc was reached at the saturated
adsorption concentration Cc that was larger than its critical micelle
concentration in 0.5 mol/L H2SO4 solution. The inhibition efficiency for zinc
slightly reduced with increasing s since C12-s-C12·2Br formed different
morphologies of the surface aggregates. The adsorption ability and the
inhibition efficiency for corrosion reduced with increment of temperature.
The pitting corrosion behavior of zinc in each of the following
solutions NaCl, NaBr and NaI were studied by potentiodynamic and cyclic
voltammetry techniques under the influence of different experimental variables
[122]. In these solutions, the anodic sweep exhibits an active/passive
transition prior to the initiation of pitting corrosion. The active dissolution
region displays one anodic peak A1. Passivity is due to the existence of a
protective film of ZnO on the anodic surface. At a certain specific
potential, pitting potential, breakdown of the anodic passivity occurs caused
by field-stimulating halide penetration into the passive oxide film at point
defects. The aggressiveness of the halide ions towards the stability of the
passive film decreases in the order: Cl > Br > I . The susceptibility of zinc to
pitting corrosion enhances with increasing the halide ion concentration and
the temperature but decreases with the sweep rate. The cathodic sweep shows
two reduction peaks C1 and C2. The more negative cathodic peak C1 is a
conjugate of the anodic peak A1 while the cathodic peak C2 is related to the
electro reduction of the pitting corrosion
53
Introductionproducts. The existence of pitting corrosion was confirmed by scanning electron
microscopy.
The use of potentiodynamic anodic polarization, cyclic voltammetry
and chronoamperometry techniques in order to study the pitting corrosion
susceptibility of a Zn electrode in KOH solutions containing KSCN as a
pitting corrosion agent[123]. Results demonstrated that in the absence of
KSCN, the anodic voltammetric response displays two anodic peaks prior
to reaching the oxygen evolution potential. The first anodic peak A1 is related
to the electro formation of Zn(OH)2. Peak A1 is followed by a wide passive
region which extends up to the appearance of the second anodic peak A2.
The latter is assigned to the formation of ZnO2. Addition of SCN ions to
the KOH solutions stimulates the anodic dissolution through peak A1 and
breaks down the passive layer prior to peak A2. The breakdown potential
decreases with an increase in SCN concentration and temperature, but increases
with an increase in KOH concentration and potential scan rate. Successive
cycling leads to a progressive increase in breakdown potential. The current /
time transients show that the incubation time for passivity breakdown
decreases slightly with increasing applied positive potential, SCN
concentration, and temperature.
The changes in the pitting corrosion current density with time on
zinc electrode concerning the concentration of both the passivating borate
and the aggressive chloride anions were followed using a simple
electrolytic cell [124]. The pitting corrosion currents started to flow after
an induction period, .This period is found to be a function of the concentration
of Cl anion, according to the relation log = – logCCl .The pitting
corrosion currents finally reached a steady-state value, which depended on the
concentration of both B4O7 and Cl anions. At a constant B4O7 anion
54
Introductionconcentration, the pitting corrosion current varied with the concentration of
Cl anion according to the relation logipi t=a1+b1logCCl . It also varied at constant
Cl anion concentration and various B4O7 anion concentration according to the
relation logipit=a2 b2logCB4O7 . The susceptibility of the passivating zinc to
pitting corrosion was found to be increasing as the temperature and pH of
the solution increased. Results are discussed on the basis of adsorption of
the aggressive anion on the passivating film, followed by penetration through
the film and incorporation in it. This undermines the oxide film and caused
pitting corrosion.
The electrochemical frequency modulation (EFM) technique provided
a new tool for electrochemical corrosion monitoring [125]. EFM is a non-
destructive technique as electrochemical impedance spectroscopy (EIS).
EFM technique was used in comparison with the traditional dc and ac
techniques. Results obtained by EFM technique were shown to be in
agreement with other electrochemical techniques. With EFM technique, a
corrosion rate can be obtained instantaneously in very short time which
makes this technique ideal in online corrosion monitoring. New synthesized
thiourea derivative named 1,3-diarylidenethiourea ( DAT ) was examined as
a new corrosion inhibitor for iron in 1 M hydrochloric acid solution.
Nonlinear electrochemical phenomenon, a potential perturbation
signal by one or more sine waves will be generated current responses at
more frequencies than the frequencies of the applied signal[126]. Current
responses can then be measured, for example, at zero,harmonic,and
intermodulation frequencies. This simple principle offers various possibilities
for corrosion rate measurements, like the intermodulation or electrochemical
frequency modulation (EFM) technique in which the potential perturbation
55
Introductionsignal consists of two sine waves of different frequencies. With this novel
EFM technique, the corrosion rate can be determined from the corrosion system
responses at the intermodulation frequencies. With the EFM technique a
corrosion rate can be obtained instantaneously, without prior knowledge of
the so-called Tafel parameters. The EFM approach requires only a small
polarizing signal, and measurements can be completed in a short period. A
special advantage of the EFM technique is its capability of inherent data
validation control using “causality factors” ( parameters introduced for the first
time in this paper). It is shown that the EFM technique can be used
successfully for corrosion rate measurements under various corrosion
conditions, such as mild steel in an acidic environment with and without
inhibitors and mild steel in a neutral environment.
The electrochemical behavior of Cu, Cu–37Zn and Zn in
benzotriazole ( BTA ) containing chloride solutions was studied and
compared using potentiodynamic, cyclic voltammetry and electrochemical
impedance spectroscopy [127]. The presence of BTA in the chloride-
containing solutions gave rise to higher breakdown potentials, significantly
higher polarization resistances and inhibited the formation of CuCl2 and
zinc-containing corrosion products. These effects were observed for pure
Cu, Cu–Zn and to a somewhat lesser extent pure Zn. The electrochemical
impedance data were consistent with the formation of a polymeric BTA-
containing layer for all three systems.
The electrochemical behavior of zinc in strong alkaline solutions
containing 8.5 M of potassium hydroxide (KOH) and polymeric organic
inhibitors was evaluated [128].The concentrations of the organic inhibitors
studies were in the range of 400–4000 ppm and included polyethylene
glycol (PEG), with a molecular weight of 600, and polyoxyethylene alkyl
56
Introductionphosphate ester acid form (GAFAC RA 600).The electrochemical studies
included anodic, cathodic, and linear polarization along with potentiostatic
studies. It was found that the inhibition properties of PEG, in the strong
alkaline solution, are by far much more efficient than the inhibition
capability of GAFAC RA 600. Surface analysis obtained with the use of
high resolution scanning electron microscopy (HRSEM) revealed different
morphology characteristic developed at the zinc surface in the presence of
the two inhibitors. A methodology employing electrochemical tests is
proposed to quickly and conveniently evaluate inhibitors for Zn in alkaline
media.
An examination of quantum chemical and corrosion inhibition
studies were carried out to be investigated [129]. Whether any clear links exist
between the results of quantum chemical calculations and the experimental
efficiencies of urea (U), thiourea (TU), acetamide (A), thioacetamide (TA),
semicarbazide(SC),thiosemicarbazide(TSC), methoxybenzaldehydethiosemi
carbazone(MBTSC),2-acetylpyridine-(4phenyl)thiosemicarbazone (2AP4PTSC)
2-acetylpyridine-(4-methyl)thiosemicarbazone (2AP4MTSC), benzointhiosemi
carbazone (BZOTSC) and benzilthiosemicarbazone (BZITSC) being corrosion
inhibitors. The quantum chemical calculations had been performed by using
DFT, abolition molecular orbital and semi-empirical methods for some amides
and thiosemicarbozone derivatives being corrosion inhibitors. The highest
occupied molecular orbital energy (EHOMO), lowest unoccupied molecular
orbital energy (ELUMO), the energy gap between EHOMO and ELUMO ( E=
EHOMO – ELUMO ), dipole moments ( ), charges on the C, O, N and S atoms,
the total energies of the molecules and the polarizabilities , the coefficients
of the development of the MO over the atomic orbital ( AO ) corresponding
to the bond between atoms which a new bond is established had been
57
Introductioncalculated. Results of quantum chemical calculations and experimental
efficiencies of inhibitors were subjected to correlation analysis.
Electrochemical frequency modulation (EFM) technique, the non-linear
behavior of a corroding system was measured. This non-linear behavior is
likely to be different for a system undergoing uniform or pitting corrosion [130].
The implementation of the EFM technique to detect pitting corrosion has been
investigated by observing the fluctuations in the so-called causality factors.
These causality factors, resulting from an EFM test and in the ideal case
having values of 2 and 3, respectively, are normally used for quality and data
validation purposes. While investigating pitting corrosion, they showed
different behavior leading to the CPT ( critical pitting temperature )
detection.
Inhibition of the corrosion of zinc in 0.01 to 0.0 4 M H2SO4 by
erythromycin was studied using weight loss and hydrogen evolution methods
[131]. The obtained results indicated that erythromycin is a good adsorption
inhibitor for the corrosion of zinc in H2SO4 solutions. The inhibition efficiency
of erythromycin increased with increasing concentration but decreased with
the increase of temperature. Thermodynamic and adsorption studies reveal
that the adsorption of erythromycin on zinc surface is exothermic, spontaneous
and is characterized with increasing degree of orderliness. The adsorption
characteristics of the inhibitor are best described by the Langmuir adsorption
model. From the variation of inhibition efficiency with temperature and the
calculated values of the activation and free energies ( which are within the
limits expected for physical adsorption ).
58
Introduction The corrosion behavior for zinc in ( HNO3 + H2SO4 ) binary acid
mixture containing ethanol amines was studied [132] . Corrosion rate
increased with concentration of acid and temperature. At constant acid
concentration, the inhibition efficiency of ethanol amines increased with the
inhibitor concentration. Value of Ga increased and inhibition decreased with
the increase of temperature. The mode of inhibition action appears to be
chemisorption.
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